Field of the Invention
The invention relates to rolling stands with working rolls having skewed axes for manufacturing seamless tubular bodies.
State of the Art
Generally, rolling mills with two skewed working roll axes, and in particular among other things modern piercing mills, used to pierce round billets in rolling mills for seamless tubes, provide a stand structure, or frame, which supports the loads generated by the deformation of the material during rolling, and roll cradles which support the working rolls. The adjustment of the existing feed angle is also provided between the two working rolls, their skewed rotation axes being oblique to each other.
Indeed, in order to allow the rolling of various products with the same rolling stand, in particular billets of various diameters, there is a practical need to modify the feed angle, which is the angle formed by the two skewed axes and which is measured with respect to the rolling axis, in order to contain the torques and forces generated in the machine.
Furthermore, modern rolling mills allow the adjustment of the distance between the working rolls. In particular, such adjustment devices are electromechanical jack screws, through which the forces of separation generated between the working rolls are discharged at the ends of the structure. These electromechanical jack screws are normally two in number for each working roll, in certain cases four jack screws are employed. The jack screws are normally assembled on the lower part of the rolling stand, fixed to the stand structure, while they are generally assembled jack screws on movable parts of the frame, tilting or shifting to allow the replacement of the working rolls and of the roll cradles.
Due to the normal elasticity of the stand structure, there may be a separation of the working rolls from each other by some mm, for example up to 3-5 mm, when they are under load. This settlement may not always be compensated for by adjusting the electromechanical jack screws, that is by pre-closing them, due to interference with the side guide systems. In particular, when the mill stand has disc guides, the latter require being positioned almost in contact with the working rolls and hence a pre-closure involves the risk of contact and damage between roll and disc. Furthermore, in the steps of rolling the head and the tail of the piece, two steps in which the separating forces are reduced, there is a thinning of the rolled section due to the automatic closing of the structure due to the elastic return of the elements caused by the decrease in the separating force.
Publication EP619150A describes a rolling mill similar to the one described above, in which the jack screws are assembled fixed on the structure and where the disengagement of the roll cradle to replace the pair of working rolls is obtained with bayonet fastening and, consequently the rolling stand is not to be equipped with a movable cap as in other solutions of the state of the art.